1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. For example, the present invention relates to a thin film transistor or a semiconductor device including an oxide semiconductor film in a channel formation region. Moreover, the present invention relates to, for example, an electro-optic device typified by a liquid crystal display panel including a thin film transistor or to a light-emitting display device including a thin film transistor and an organic light-emitting element. Furthermore, the present invention relates to an electronic appliance in which such an electro-optic device or a light-emitting display device as above is incorporated as a component.
The semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics; semiconductor elements, electro-optic devices, semiconductor circuits, and electronic appliances including semiconductor elements are all semiconductor devices.
2. Description of the Related Art
In recent years, active-matrix display devices (such as liquid crystal display devices, light-emitting display devices, or electrophoretic display devices) in which a switching element including a thin film transistor (hereinafter referred to as a TFT) is provided for each of display pixels arranged in matrix have been actively developed. In the active matrix display devices, a switching element is provided for each of pixels (or each of dots), and thus, there is such an advantage that the active matrix display devices can be driven at lower voltage than that of passive matrix display devices in the case where the pixel density is increased.
Meanwhile, there are a variety of kinds of metal oxides intended for many uses. Indium oxide is a well-known material and is used for a transparent electrode material necessary for a liquid crystal display or the like.
Further, some metal oxides have semiconductor characteristics. The metal oxides having semiconductor characteristics are one kind of compound semiconductor. The compound semiconductor is a semiconductor formed by two or more kinds of atoms bonded together. In general, metal oxides are insulators; however, it is known that metal oxides are semiconductors depending on the combination of elements included in the metal oxides.
For example, among metal oxides, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are known as the metal oxides having semiconductor characteristics. The metal oxide as above is used for a transparent semiconductor layer serving as a channel formation region in a thin film transistor (as disclosed in Patent Documents 1 to 4 and Non-patent Document 1).
Examples of metal oxides include not only an oxide of a single metal element but also an oxide of a plurality of metal elements. For example, InGaO3(ZnO)m (m is a natural number) which is a homologous compound is a known material (Non-patent Documents 2 to 4).
Then, it has been confirmed that such an In—Ga—Zn-based oxide as above is applicable to a channel layer of a thin film transistor (Patent Document 5 and Non-patent Documents 5 and 6).
Patent Documents 6 and 7 and the like disclose a technique applied to a switching element or the like of an image display device, in which a thin film transistor including such an oxide semiconductor film is formed over a light-transmitting substrate. Further, Patent Documents 8 and 9 disclose an attempt for manufacturing a light-transmitting thin film transistor by using a light-transmitting conductive film for each of a gate electrode, a source electrode, and a drain electrode. Patent Document 10 discloses a technique in which a gate insulating film is processed in an oxygen atmosphere to improve the characteristics of an interface with an oxide semiconductor film.